Figure 4

Experimental set-up for testing of stepping and grasping reactions. Schematic drawing showing the experimental set-up for the balance testing (performed at baseline and after the six weeks of training). The platform is semi-enclosed, with walls to the front and sides of the subject; for illustration purposes, the left wall has been rendered semi-transparent. The motion platform is controlled by a motor (located underneath the surface) to move unpredictably in one of the four directions shown. The cable-pull system also delivers unpredictable perturbations in these four directions. Four cables are attached to the belt at the waist, and are routed via a system of pulleys to a weight-drop apparatus that is located behind the front wall of the platform (out of the view of the subject). Prior to each trial, the experimenter manually connects one of the four cables to the weight. An electromagnet is then used to initiate the weight drop. When the weight is dropped, the subject is pulled unpredictably in one of the four directions, depending on which cable is attached to the weight. Prior to perturbation onset, an equivalent amount of slack (~2–4 cm) in each cable is maintained via a locking mechanism; hence, subjects are free to sway to an equal extent in any direction and cannot detect which cable is attached to the weight. During the testing of grasping reactions, a handrail (not shown) is mounted to the right of the subject (25% of body height from midline; height of rail = 55% of body height) and foam blocks (40cm high) are placed around the feet to deter stepping (similar to Figure 2B).